The present invention relates generally to a charging device for charging a moving member as it travels pass the device. More particularly, the present invention relates to a corona charging device adapted for use in electrophotographic systems, wherein the corona charging device includes a plurality of corona discharge wires and a grid electrode, which is interposed between the wires and a photosensitive drum (hereinafter referred to as the PC drum).
It is known in the art to employ a plurality of discharge wires in electrophotographic systems so as to increase the efficiency of charging devices.
A typical example of the known charging devices is shown in FIG. 9, in which a plurality of discharge wires (in FIG. 9 they are represented by two wires a and b) are equally spaced from a PC drum, the wires being extended in parallel with each other. There is provided a grid electrode G between the discharge wires a, b and the PC drum, the grid electrode being made of stainless steel meshwork or lath. The grid electrode G is extended in parallel with a plane including the discharge wires a, b. FIG. 10 shows another example which is provided with a grid electrode G.sub.W extended in parallel with the PC drum. Each charging devices of FIGS. 9 and 10 are provided with a covering plate S above the discharge wires a, b.
The ultimate surface potential on the PC drum depends upon the porosity or degree of openness of the grid electrode, the grid voltage or the varistor voltage induced when the grid electrode is grounded through the varistor, and the distance between the grid electrode and the PC drum. Especially it is important what a distance the grid electrode and the PC drum are spaced from each other immediately below the discharge wires.
In FIG. 9 the distance d between the grid electrode and the PC drum immediately below the discharge wires a, b is larger than the distance .alpha. therebetween below a middle point between the two wires a and b. the d&gt;.alpha. relationship results in the low ultimate surface potential. As a solution the grid electrode is placed closer to the PC drum. However, the closer placement of the grid electrode to the drum is likely to cause leaks therebetween.
The disadvantage of the device of FIG. 10 is the difficulty of extending a plurality of grid wires w at equal intervals and under equal tension, and maintaining an equal distance from the PC drum. The assembling of the device requires an experience and skill, which reflects in the production cost.